In general, the inverter power source control circuit is so designed that an alternating current output from an alternating current power source is converted to a direct current output and, in order to convert this direct current output to a desired alternating current output that is to be supplied to a high frequency heating apparatus, a start signal such as, for example, a PWM control signal is applied to a gate of a switching element. A control circuit for controlling the entire inverter power source is so designed as to detect one or more abnormalities occurring in electric current, voltage and/or temperature in the high frequency heating apparatus to suppress the occurrence of such abnormalities and/or to halt oscillation of the inverter power source. In most cases, this control is performed with the use of a microcomputer. One conventional inverter power source control circuit is shown in FIG. 6.
Referring to FIG. 6, the microcomputer MC detects an abnormality occurring in an input current through a current transformer 6, an abnormal temperature in the high frequency heating apparatus through a temperature sensitive element 15, and an abnormality occurring in a drive voltage of a switching element 7.
Also, in the inverter power source used as an electric power source of a high frequency heating apparatus such as, for example, a microwave oven, the switching element referred to above is generally employed in the form of an insulated gate bipolar transistor (IGBT) element. This IGBT element is a one-chip element, in which a MOS-FET (metal-oxide semiconductor field-effect transistor) and a bipolar transistor are combined together and therefore has combined features of a low electric drive power and a high speed switching characteristic, both exhibited by the MOS-FET and a low resistance exhibited by the bipolar transistor.
On the one hand, the control circuit is known, which is in the form of an analog circuit, not a digital circuit utilizing a microcomputer, for detecting an abnormal electric current, an abnormal electric voltage and an abnormal temperature individually and for subsequently halting the oscillation of the inverter power source. See, for example, the Japanese Laid-open Patent Publication No. 2002-354659.
Detection of the abnormal electric current is generally intended to detect an abnormality occurring in the input electric current when the input electric current lowers as a result of the power failure such as brought about by breakage of the magnetron, and unless a trouble shooting is successfully performed, the detection is sustained. However, with the conventional analog circuit, since an abnormality occurring in the input electric current is detected even when the oscillation is halted because of the abnormal voltage and/or the abnormal temperature, the abnormal current detection is required to be reset when operation is to be restarted upon removal of the abnormal voltage and/or the abnormal temperature, making it difficult to accomplish an automatic resetting.
On the other hand, in the inverter power source for a microwave oven, which utilizes an IGBT element, although the IGBT element has such an excellent characteristic as hereinbefore described, it has a drawback due to its structure, in that it may function erroneously when applied with an excessive electric current and is possibly apt to breakdown as the IGBT element is more sensitive to influences brought about by overheating than any other elements. In view of this characteristic, such abnormalities must be monitored for safety purpose, i.e., to protect from the excessive current and, even with other component parts such as, for example, a magnetron and PWM, the occurrence of an abnormality must also be monitored for safety purpose. Accordingly, the need is widely recognized that the abnormal temperature, voltage and current must be comprehensively monitored to accomplish the protection. In such case, it may occur that the abnormality monitoring level is required to modify during the maintenance and/or revision of the safety standards. Also, even at the time of shipment, the monitoring level would be required to modify depending on requirements in a country in which the inverter power source is used.
However, considering that a microcomputer is used for controlling a protection device for the conventional inverter power source and the monitoring level is programmed in the computer, modification of the monitoring level is generally difficult to accomplish. Also, where the microcomputer is used, there is a problem in that the cost of manufacture of devices does not decrease.